There are both screw compressors which operate in a dry running operating mode and screw compressors which operate in an oil-flooded operating mode. In the latter screw compressors, oil is used both as a lubricant and as a sealant during the compression operation itself. In order, however, to avoid any undesired oil ingress into a refrigeration unit which is supplied with compressed refrigerant by the compressor, an oil separator is as a rule provided on a high pressure side of screw compressors of this type.
According to the prior art, an oil separator of a semi-hermetic compact screw compressor consists of two spaces separated by a demistor made from knitted metal mesh. Compressed gas/oil mixture (refrigerant/oil mixture) passes from a pressure pipe into the first space, in which a pre-separation takes place, in particular at a bounding wall of the first space. Afterward, the gas flows with a reduced oil content and possibly oil which has already been separated and was recently entrained by the gas flow out of the first space through the demistor into the second space. The oil content in the gas is reduced further in the demistor. The remaining oil separation takes place in the second space.
In the described construction, however, the lower part of the demistor knitted mesh lies in the oil which has already been separated, and part of the oil is pulled upward by way of capillary forces and mixes again with the gas. Furthermore, in a manner which is dependent on operating conditions, the gas and oil phases in the knitted mesh have a different flow resistance and accordingly the oil level in the first and in the second space is of different height. As a result, the gas flows partially through the oil which has already been separated, and therefore the phases are mixed again.